For some years, different types of radio networks for wireless communication have been developed to provide radio access for various wireless devices. The radio networks are constantly improved to provide better coverage and capacity to meet demands from subscribers using increasingly advanced services and devices such as smartphones and tablets, which may require considerable amounts of bandwidth and resources for data transport in the networks. A limiting factor for capacity in a radio network is the amount of available radio resources, e.g. in terms of time, frequency bandwidth and transmit power. The capacity of a radio network can be improved by utilizing any available radio resources as efficiently as possible, e.g. by reducing or minimizing the amount of signaling between the devices and the network in order to use the radio resources for communication of payload data and any mandatory messages.
In this disclosure, the term “wireless device” is used to represent any communication entity capable of radio communication with a radio network by sending and receiving radio signals, such as e.g. mobile telephones, tablets, laptop computers and so-called Machine-to-Machine, M2M, devices. Another equivalent common generic term in this field is “User Equipment, UE” which could also be used instead of wireless device throughout this description.
Further, the term “network node”, is used herein to represent any node of a radio network that is operative to communicate radio signals with wireless devices. The network node in this disclosure could also be referred to as a base station, radio node, e-NodeB, eNB, NB, base transceiver station, access point, etc., depending on the type of network and terminology used. Furthermore, “network node” may also refer to an entity or equipment controlling the unit that is actually communicates radio signals with wireless devices. This may e.g. be a base band unit controlling a remote radio unit or a node controlling radio base stations, e.g. a radio network controller. It is further assumed that the radio network discussed herein provides radio coverage in different cells which are served by corresponding network nodes.
In order to improve capacity and performance in the radio network, various features can be employed that are intended to make the radio communication more efficient in terms of resource usage. Furthermore, it is desirable to reduce energy consumption in the network as well as the amount of interference generated by transmissions made by network nodes and wireless devices, which in turn could improve both capacity and performance. It is for example desirable to limit the broadcasting of system information from network nodes, sometimes generally referred to as the “broadcast layer”.
In order to access a radio network, a wireless device has to acquire certain parameters which are generally broadcasted in the cells of the radio network in so-called system information, SI. Throughout this description, the term “cell” is used to represent any area in which a network node provides radio coverage for wireless devices. Hence, this description is not limited to cellular networks and it may be valid and useful for any type of radio network in which various network nodes provide radio coverage. The description may also be valid for radio networks in which network nodes use beamforming to cover respective areas with radio transmissions.
The network nodes thus broadcast a certain amount of information in each cell including synchronization signal(s) such as a Primary Synchronization Signal, PSS, and a Secondary Synchronization Signal, SSS, as in a 3GPP Long Term Evolution, LTE, system, which signals can be used by the wireless device to obtain frequency and time synchronization with respect to symbol and frame, respectively. These LTE signals also encode the Physical Cell Identity, PCI. After synchronization and PCI detection, the wireless device is capable of performing channel estimation using broadcasted cell specific reference signals C-RSs, and finally decode the broadcasted system information. The PSS/SSS and C-RSs are constantly transmitted by the LTE network and they can thus be used by wireless devices to synchronize and perform channel estimation.
In LTE networks, the broadcasted system information is structured by means of System Information Blocks, SIBs, which include a Master Information Block, MIB, a System Information Block Type 1, SIB1, and a System Information Block Type 2, SIB2. The MIB includes a limited number of the most frequently transmitted parameters which are essential for a wireless device's initial access to the network. The SIB1 contains parameters needed to determine if a cell is suitable for cell selection, as well as information about the time-domain scheduling of the other SIBs. The SIB2 includes certain common and shared channel information. Further SIB types have also been defined and the list of SIBs has been expanding over the years and it is expected to continue increasing. The system information is constantly broadcasted in the network with some periodicity, depending on the type of information. It can thus be readily understood that much energy and radio resources would be consumed when broadcasting all this information throughout the network.
It has been proposed that the same system information should be broadcasted at regular intervals in a synchronized manner over a relatively large area, so as to reduce and minimize the total broadcast duration and avoid interference. The goal is to transmit as little as possible apart from data transmissions to individual devices. If there are no ongoing data transmissions in a particular cell or area, the network nodes of that area can turn off their transmitter between the broadcasting occasions to save power and also to avoid interference in adjacent areas.
However, different network nodes may need to apply different system information configurations locally in different cells or areas, depending on the current conditions which may include: the network node's capacity, its carrier frequencies, size of the cell, expected frequency of access attempts, expected traffic volume, current ongoing data communications, the number of wireless devices present in the cell or area, the number of access messages currently being transmitted, and so forth. A network node may even apply different system information configurations in different cells served by the same network node.
If aggregated system information containing all possible system information configurations is broadcasted over a large area and stored in the wireless devices, a selected system signature pointing to a specific system information configuration can be transmitted locally in a cell, i.e. smaller area, to indicate that this configuration is valid in this cell or area, so that any wireless devices in the cell or small area can use the system signature as a “key” to extract the system information configuration from the stored system information which is valid in this particular area.
Any wireless devices present in this area are thereby able to derive a relevant system information configuration from its stored system information based on the system signature, which may be a signature index sequence, SSI, or a PCI, which is thus transmitted locally in the area as a reference to the system information configuration to be used in this area.
FIG. 1 illustrates an example of a communication scenario in a hierarchical network structure comprising various network nodes including a network node 100 providing radio coverage over a relatively large area C1 and a plurality of further network nodes 102 providing radio coverage over much smaller areas C2 substantially within the area C1. The network node 100 broadcasts system information over the large area C1 which can be read by any wireless devices D present in the area C1. Alternatively, the system information may be broadcasted in a synchronized manner by several network nodes, not necessarily arranged hierarchically, which together cover a large area. Typically, system information needs to be broadcasted with enough reliability so that it can be received properly by any wireless device present within the large radio coverage area C1. Network nodes transmitting system information may aggregate and transmit system information and such aggregated system information may be provided in a so-called Access Information Table, AIT.
Each network node 102 transmits a system signature, e.g. SSI, locally in the respective area C2 to indicate different system information configurations in different areas C2. There are several possible ways of communicating the system information AIT and the system signatures SSI in a radio network, including all network nodes transmitting both the AIT and respective SSIs, or some network nodes transmitting the AIT and other network nodes transmitting the SSIs, or one macro node transmitting the AIT and smaller nodes under the macro node transmitting the SSIs, and so forth.
FIG. 2 illustrates that system information configurations are broadcasted in an AIT with a certain periodicity over time and the system signature is signaled in an SSI with another periodicity that may be shorter than the periodicity of the AIT. It is also possible to transmit the AIT and SSI together at the same time. The AIT may thus be broadcasted over a large area, e.g. by a network node such as the macro node 100 over area C1 in FIG. 1, and different SSIs may be transmitted in smaller areas within the large area, e.g. by the smaller network nodes 102 over areas C2.
Once a wireless device has received a system signature such as an SSI in a particular area, it will be able to derive an associated system information configuration from the AIT which it has previously received and stored. This system information configuration and its parameter settings can then be used by the wireless device for accessing the radio network in that particular area. The SSI, or generally the system signature, is effectively a “key” for deriving the correct and valid system information configuration from the broadcasted system information configurations, e.g. the AIT. When the wireless device moves to another area, it will receive another SSI and derive another system information configuration accordingly which is valid and should be used in the new area.
However, it is a problem that a network containing a large number of cells would need to employ a large number of unique system signatures to enable the use different system information configurations in the cells throughout the network. If the number of system signatures is large, their unique identities would require a great number of bits resulting in too high complexity in the wireless devices when trying to decode the transmitted system signatures. It is therefore necessary to limit the number of unique system signatures and hence reuse them in several cells across the network. It may also be that the same system signatures comprised of a limited number of bits are used by two different networks to reduce complexity, although the system signatures typically have different meanings, i.e. they are associated with different system information configurations, in the two networks.
Another problem is that in order to limit the number of bits that are transmitted over the air for energy efficiency purposes and reduced complexity, the system signatures SSIs must be kept short. If they have too many bits, e.g. in the size of MIB and SIB-1 and SIB-2 in LTE, the whole purpose of reducing energy consumption by transmitting the AIT and the SSIs as keys is spoilt. It is thus of great interest to keep the number of system signatures SSIs, and the number of bits therein, as low as possible by reusing them in multiple areas as described above.
As a result, one system signature that is used in one limited area as a reference to a certain system information configuration may be reused in another area as a reference to a different system information configuration. If so, there is a risk that a wireless device having saved a system information configuration when present in one area may use that system information configuration erroneously for accessing another area that reuses the same system signature for another system information configuration. The wireless device may even unwittingly try to access a new unpermitted network using a system information configuration acquired in a previous network.
In either case, such erroneous access attempts may be harmful to the network and other wireless devices by creating collisions and uncontrolled interference when using the wrong system information configuration. Some examples of such harmful behavior of the device include using the wrong radio resources for random access, using too high transmit power, using a non-allowed random access preamble, and so forth. The performance in the wireless device will naturally also suffer since it is not able to access the network wasting both time and battery power.
This situation is illustrated in FIG. 3 where a wireless device 300 first acquires system information configurations AIT-1 when present in one area 302 and further receives a system signature SSI 1 when transmitted from a serving network node 304. According to AIT-1, SSI 1 points to a certain system information configuration denoted “content X” which is stored by the wireless device 300 according to regular procedures. It should be noted that the same system signature may be transmitted over more than one area or cell as long as the same system information configuration is applicable in those areas. For example, the SSI does not even have a cell identity encoded.
The wireless device 300 then moves to another area 306 in which other system information configurations AIT-2 are valid. For example, each area 302, 306 may comprise one cell or a group of cells and the examples herein are not limited in this respect. The same system signature SSI 1 is reused in area 306 but points to another system information configuration denoted “content A” according to the system information AIT-2, where content A is different from content X, e.g. with respect to various radio parameters for radio access. When the wireless device 300 receives the same system signature SSI 1 transmitted from a serving network node 308 in area 306, the device 300 retrieves the previously stored content X, according to regular procedures, and uses the radio parameters according to content X for accessing the network node 308. Using content X instead of the correct content A will thus not be successful and may further be harmful in area 306, as explained above.